Problem: Determine how many solutions exist for the system of equations. ${-x-y = -9}$ ${y = -x+4}$
Explanation: Convert both equations to slope-intercept form: ${-x-y = -9}$ $-x{+x} - y = -9{+x}$ $-y = -9+x$ $y = 9-x$ ${y = -x+9}$ ${y = -x+4}$ Just by looking at both equations in slope-intercept form, what can you determine? ${y = -x+9}$ ${y = -x+4}$ Both equations have the same slope with different y-intercepts. This means the equations are parallel. ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$ ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$ Parallel lines never intersect, thus there are NO SOLUTIONS.